U.S. Pat. No. 5,071,403 discloses an overflow protection which consists of an air permeable filter interposed in the suction pathway between the breast hood and the pump. This filter is, however, substantially impermeable by milk. If milk reaches the filter, e.g. due to the fact that the collecting space is already completely filled with milk, the pores of the filter material are becoming closed when milk wets it. The disadvantage of this approach is that the filter, after contacting milk, can, in general, no longer be used and, therefore, has to be replaced. Moreover the filter constitutes a not inconsiderable resistance against suction air flow resulting in the necessity of stronger dimensioning the pump and in a higher energy consumption. Just the latter is undesirable when used in battery operated pumping appliances.
Clearly, many check valves, mostly of the ball type, are known in the art and could be used as a protection against some entering liquid. However, there is a certain time gap from the moment when the liquid enters the valve housing up to full closure of the valve. In most applications, this does not do any harm. However, when milk enters a space where it can hardly be removed, it would form a nutritive agent for bacteria which, just in an application for the human body and, above all, in breast pumps, should be avoided by all means.